Add a note about Stellar testnet vs mainnet to .env.example

[Sulex45]

.env.example has STELLAR_NETWORK and HORIZON_URL variables but no explanation of what values to use for testnet versus mainnet, or where to find the correct Horizon URLs. Add inline comments above each variable explaining the testnet and mainnet values, and include a link to the Stellar documentation for finding public Horizon endpoints.

Acceptance Criteria: .env.example has clear comments above STELLAR_NETWORK and HORIZON_URL showing both testnet and mainnet values. A link to the Stellar docs for Horizon endpoints is included.

[chisomvictorcv-sketch]

@chisomvictorcv-sketch has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello maintainer, i have gone through the issues description and I have good knowledge on it kindly assign this to me.

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[devcarole]

@devcarole has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I would like to work on this issue

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Congratulations, @devcarole! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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This issue has been completed by @devcarole as part of the Stellar Wave Program's 6th Wave 🥳

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